WASHINGTON (Reuters) - Nearly half of Americans believe that the United States should not be required to defend NATO allies from attack if they do not spend more on defense, according to a Reuters/Ipsos poll conducted after U.S. President Donald Trump’s trip to Europe.

A NATO flag is seen ahead of the Summit in Brussels, Belgium, July 10, 2018. REUTERS/Reinhard Krause

America’s NATO allies have been shaken by Trump’s harangue against them in Brussels over their under-spending on defense, and his unrestrained effort to curry favor with Russian President Vladimir Putin during their meeting in Helsinki.

A commitment to collective defense is the bedrock of the NATO alliance. It was founded in 1949 to contain the military threat from the Soviet Union. Its charter’s Article V stipulates that an attack on one ally is an attack against all.

NATO diplomats have told Reuters they worry that Trump’s portrayal of NATO as an alliance in crisis has raised concern that the U.S. president’s nagging criticism might erode U.S. public support and risk America’s commitment to collective defense. In the poll, 49 percent of respondents said the United States should not have to uphold its treaty commitments if allies do not spend more on defense. Another 18 percent said they were not sure if the United States had to uphold those commitments.

A third of those polled said did not agree with the idea of linking America’s treaty commitments to an increase in allies’ defense spending.

Two thirds of registered Republicans said the United States should not have to uphold its treaty commitments, while almost four out of 10 Democrats held that view.

NATO, which has viewed Moscow as a threat to European stability in the wake of Russia’s 2014 annexation of Crimea from Ukraine, relies on the United States’ military superiority to face down a host of threats on Europe’s borders. That includes a resurgent, nuclear-armed Russia and militant attacks.

Trump lambasted allies for failing to meet a spending target of 2 percent of a country’s economic output. He also has claimed that the United States pays for 90 percent of European security, which NATO data shows is incorrect.

While U.S. defense spending makes up 70 percent of combined allied governments’ military budgets, just 15 percent of U.S. expenditure is spent in Europe on NATO-related defense.

After traveling to Belgium and Britain, Trump stunned the world in Finland on Monday by failing to hold Putin accountable for meddling in the 2016 U.S. election as the two stood side-by-side. Trump said on Tuesday he misspoke.

The Reuters/Ipsos poll gathered responses from 1,011 registered voters throughout the United States, including 453 Republicans and 399 Democrats. The poll has a credibility interval, a measure of precision, of 4 percentage points.

The poll also found that while more than half of Americans disapprove of the way Trump is handling relations with Russia, his performance at the Helsinki summit did not seem to have an impact on his overall approval rating.